Bloch line dynamics within magnetic domain walls

Touko Herranen

Research output: ThesisDoctoral ThesisCollection of Articles

Abstract

Magnetic domains are uniformly magnetized regions within a ferromagnet separated by magnetic domain walls. The internal degrees of freedom of a domain wall can be excited by applying a magnetic field greater than so called Walker field. As a result the domain wall velocity experiences the Walker breakdown, an abrupt drop of the average velocity, and the magnetization of the domain wall starts a cyclic rotation. If this mechanism is triggered in a domain wall with a dimension greater than a material characteristic Bloch line width, the excitations become non-uniform, which results in nucleation of domain walls within the domain wall called Bloch lines. The dynamics of domain walls in disordered media have been studied extensively using various computational methods as well as experimentally. In this doctoral dissertation we use a micromagnetics software to simulate the Bloch line dynamics and the effects of Bloch lines on domain wall dynamics in samples with perpendicular magnetic anisotropy. In Publication I we study the domain wall dynamics in CoPtCr thin films with different widths. We observe nucleation of Bloch lines within domain walls in disordered and perfect samples when driven with a magnetic field higher than the Walker field. We construct a geometry to confine the domain wall between two notches, and use it to study Bloch line dynamics under an in-plane field. Finally we demonstrate the performance potential of an electrical current operated Bloch line memory. In Publication II we study the effects of boundary conditions and thickness effects on the domain wall dynamics in a magnetic garnet, and we find they determine the internal dynamics allowed for the magnetization of the domain wall. The sample thickness limits the maximum achievable stable velocity before the breakdown. The velocity limit is also found to be related to the spatial width of Bloch lines. In Publication III we use a micromagnetics approach to study the Barkhausen effect and avalanche statistics in a thin Pt/Co/Pt multilayer. The domain wall is driven using a quasistatic constant velocity. The novel approach enables us to determine magnetization of the domain wall segment where avalanches are triggered. Internal magnetization dynamics show that during avalanches the activity of in-plane magnetization, i.e. the Bloch line motion, is higher than the activity related to the domain wall motion. Avalanche size and duration distributions obtained from the activity signals follow power law scaling, and the corresponding features extracted from the domain wall velocity show no significant difference. The analysis also shows that the results obtained using micromagnetic simulations are close to the values expected from a simpler model describing a short-range elastic string in a random medium.
Translated title of the contributionMagneettisen seinämän Bloch-viivojen dynamiikka
Original languageEnglish
QualificationDoctor's degree
Awarding Institution
  • Aalto University
Supervisors/Advisors
  • Alava, Mikko, Supervising Professor
  • Laurson, Lasse, Thesis Advisor
Publisher
Print ISBNs978-952-60-8133-5
Electronic ISBNs978-952-60-8134-2
Publication statusPublished - 2018
MoE publication typeG5 Doctoral dissertation (article)

Keywords

  • micromagnetics
  • domain wall dynamics
  • Bloch lines
  • Barkhausen noise

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